On a cold night last December, a white limousine stopped near Il Boschetto restaurant in Yonkers. Two large men stepped out and hurried through the restaurant's front door.

Both men were suspected of being mob racketeers. One, Joseph Datello, has been identified as an inducted member of the Lucchese crime family. The other, Sean J. Richard, was the son-in-law of John Riggi, the imprisoned boss of the New Jersey- based DeCavalcantes. Mr. Richard had recently been in a car accident. He wore a bandage on his left arm.

In the restaurant, Steven L. Crea, who has been identified as the Luccheses' acting boss, greeted them from the first stool at the bar. For about an hour, the three men talked about bribery and extortion, the authorities say. Mr. Richard now remembers that Mr. Crea was angry. He speculated darkly about a man he thought might be an informer.

When the meeting ended, Mr. Richard dropped Mr. Datello off at his home on Staten Island, as he had after many meetings before. Then he broke from the routine.

He directed his driver to a motel parking lot and switched cars, taking a seat in a sedan with another group of men. Reaching into his sweatsuit, he unrolled the bandage and removed a hidden recording device.

Sean Richard had just betrayed the man he says was his Mafia boss. Mr. Crea had guessed wrong.

In the end, it was a fellow boss's son-in-law, worried that his future held either jail time or a violent death, who had turned against him. In the parlance of mobsters, Mr. Richard had become a rat.

"These are not the kind of circumstances you want to gain fame and notoriety for," Mr. Richard said in a recent interview. "But I didn't have many options, and that's what I am."

Mr. Richard, who agreed to several interviews while in hiding, is the mob's latest embarrassing, tell-all turncoat. He is an eager raconteur.

On Wednesday, Robert M. Morgenthau, the Manhattan district attorney, unsealed racketeering indictments against Mr. Crea and 37 other defendants, accusing them of participating in a criminal enterprise that drove up the prices of construction projects throughout the city. The indictment identifies Mr. Crea as the acting boss and Mr. Datello as a soldier in the Lucchese crime family.

As the prosecutions go forward, they will rely on years of investigation and wiretaps by the New York Police Department. And they will rely on Mr. Richard, who agreed to plead guilty to attempted enterprise corruption in exchange for a new identity and five years' probation.

In return, he plans to testify with zeal about how the Luccheses, through persistence and payoffs, maintained a grip on construction projects at a time when the Mafia was supposedly in decline.

Dozens of contractors, union officials and mobsters are said to be vulnerable to his testimony, which will recount, in detail, bribes doled out in bars and restaurants, coercion at construction sites and the Mafia's aggressive, if somewhat bumbling, management of a sprawling criminal scheme.

"He is a godsend," said a lawyer who is familiar with the case.

It is a remarkable reversal. As Mr. Riggi's son-in-law and the father of two of the boss's granddaughters, Mr. Richard, 35, is literally married to the mob. And he is displaying an almost gloating disrespect. In interviews, he went beyond details of labor corruption. He spoke with disdain for the mobsters he once served.

He described Emmanuel Riggi, one of his brothers-in-law and a man prosecutors describe as a member of the DeCavalcante crime family, as "so fat he breaks chairs at every family function." He said John Riggi, the feared crime boss, "ought to thank me for feeding his useless kids." He said Mr. Datello, his 6-foot- 5 former partner, was "a big idiot, like Frankenstein."

"I used to say, `Hey, Joey, I've got to take you in to get the bolts for your neck,' " Mr. Richard said.

Some of Mr. Richard's former Mafia associates seem astounded at the breadth and tone of his confessions. A few, through lawyers or associates, declined to comment, including John Riggi, his sons and Mr. Datello, who has eluded arrest so far.

Others, through associates or lawyers, said Mr. Richard was little more than a wild exaggerator and a serial manipulator who drew attention to the Luccheses through a foolishly flamboyant style. "This guy was a wannabe, and he conducted business that was not authorized by organized crime," said a person who is familiar with the Lucchese family.

Mr. Richard is a gritty man and emanates the blunt, coarse demeanor of a thug. He is utterly unlettered. He says "mob fractions" when he means "mob factions." But he is also capable of warmth and charm, and people who know him say he is very smart, a man who passed easily through many roles until arriving at his latest: prosecution witness.

"He's a chameleon," said a friend of nearly 20 years who spoke only on the condition of anonymity. "He became a gangster better than one of them, and he thought circles around them while he did it."

For his first 30 years, Mr. Richard was an unknown, a laborer from the Bronx who trained as a union apprentice and, by his mid-20's, started a small subcontracting company in New Jersey. His insider's tour of the mob did not begin until 1995, when he began dating Sara M. Riggi, the daughter of the DeCavalcantes' boss.

The DeCavalcantes are the smallest of the region's Mafia families, and considered by law enforcement to be the weakest, often relying on cooperation with the five families in New York. But they have long specialized in labor rackets: extorting contractors in exchange for labor peace, or replacing unionized employees with nonunion workers and then spiriting away savings on wages or fringe benefits.

Mr. Richard became fascinated with Mr. Riggi, whom he said he first visited in the Federal Correctional Institution in Cumberland, Md., in 1996, when the boss was serving a sentence for extortion. Mr. Riggi, 75, appeared in prison khakis, yet somehow managed, Mr. Richard said, "to have an almost presidential bearing." He sat briefly with Mr. Richard and Ms. Riggi before sending his daughter to a vending machine.

"As soon as she left, he looked at me and said: "Who you with? Who you indentured to?" Mr. Richard said. "I said: "Nobody. I don't answer to nobody.' And after we talked for a while, we hit it off."

Unlike Mr. Riggi's three sons, who law enforcement officials say were unable to run the rackets (Mr. Richard calls them "goofballs, three guys you wouldn't take miniature golfing"), his son-in-law seemed smart and aggressive. Soon, Mr. Richard's legitimate business was a sideline; labor rackets were his game.

He and law enforcement officials said that in 1996 he began circulating his father-in-law's instructions to corrupt New Jersey locals and pursuing larger plans. "Sean was a significant player," said Robert T. Buccino, who retired last month as deputy chief of New Jersey's Organized Crime Bureau. "He was more trusted than the sons were."

By 1997, Mr. Richard established a link with the Luccheses, working for Mr. Datello, who prosecutors say also specialized in labor rackets. In one of his schemes, Mr. Richard and prosecutors assert, he and an associate bribed Michael Forde, the president of Local 608 of the United Brotherhood of Carpenters and Joiners, to help limit the number of unionized carpenters at a Park Central Hotel renovation project.

He said the first payment, $10,000 in cash, was made over afternoon beers at a Hooters on West 56th Street. "I gave him the ten thousand, and he says, `You know, I really shouldn't be drinking beers while I'm working; the union is cracking down on that,' " Mr. Richard said. "I said: `You're worried about the beers? What do you think your guys would think about that ten thousand you just took?'"

Through his lawyer, Mr. Forde said the meeting with Mr. Richard never took place.

As he worked for Mr. Datello, Mr. Richard lived a high roller's life, canvassing Manhattan and spending freely from wads of $100 bills. He bought pedigreed Rottweilers. He leased a new luxury car every few months. He bought a $320,000 home on two acres in Holmdel, N.J., putting 145,000 down. The house had a sauna, a Jacuzzi, an in-ground pool and separate yards: one for his two infant daughters, the other for his menacing-looking black dogs.

By 1999, he had become a fixture at the Oak Room at the Plaza Hotel, where the manager, Patrick Littlejohn, remembers that he sat in a corner booth and ordered heaping plates of chilled seafood and seared prime rib, washing both down with 1982 Lafite-Rothschild. Sometimes he stood by the piano, singing Frank Sinatra or Perry Como songs.

He also passed long hours at the Paradise Club, a nightclub near the Empire State Building, where he tipped the striptease dancer he fell in love with as much as $1,000 a night, he and the dancer both say. "If I dropped dead tomorrow, I lived, baby, I lived," Mr. Richard said.

He and the authorities also say he attended the meetings of a secret three-member construction panel that the Luccheses used to plan rackets, mediate disputes and divide spoils. By his telling, these meetings illustrated the paradox that the latter-day Mafia has become.

On one hand, the mob was resourceful enough to cheat the construction industry of millions of dollars. On the other, many members and associates were almost pathetic: grown men who groveled to get jobs as flagmen at road crews, or who talked relentlessly on cellular phones about secret meetings, all while the detectives were listening.

"This was supposed to be a secret society, but we'd get 20, 25, 30 guys turning up, all asking for something," Mr. Richard said. "It was a joke."

He recalled one panel meeting last year at Little Charlie's Clam Bar on Kenmare Street in Manhattan, for which, he said, two dozen mobsters and associates arrived in flashy cars, lining the block with Cadillacs and Lincolns. Later, he learned that the police had watched the entire show. "It was kind of comical," Mr. Richard said. "The detectives were right across the street."

Mr. Richard's life began to unravel last June, when the police raided several dozen homes and offices in New York, New Jersey and Connecticut, searching for records documenting the suspected rackets. One of the targets was the Linden, N.J., office of S & S Contracting, the business he had formed with his wife.

The raids filled him with dread. "It was the beginning of the end," he said. "For them to get 40 warrants in three states? Come on. That's real."

Over the next five months, he fell into a funk. He began to use cocaine, marijuana and heroin, he said, and drank more heavily than usual. He spent even more money, dipping into the weekly cash tribute he said he was supposed to pay Mr. Datello as a member of his crew.

He also began to think about becoming the state's witness, in part because friction within the Luccheses rose after the raids, and he worried that Mr. Datello had been given orders to kill him. The fear, he said, intensified late last fall when he was told to wait outside the Tick Tock Diner in Clifton, N.J., to meet Dominic Truscello, a Lucchese capo. Mr. Datello told him he would be picked up in a van.

"I said, `What do you guys need a van for?' " Mr. Richard recalled. "A van is never a good sign in this business."

He was so unnerved, he said, that he armed himself, carrying a pistol in his belt. The van ride passed without violence, but during the meeting that followed, he said, Mr. Truscello stared icily at him and said, "What are your sins?" Mr. Richard interpreted the question as a sign that he was marked for execution. He defected soon thereafter.

Now that the case is public, lawyers and associates of the Luccheses say that the state's star witness is a traitor on many levels.

Mr. Richard is now in hiding with his girlfriend, the stripper, who performed under the name Lola. His wife, Sara Riggi, has not seen him in months. She has been left in disarray, alone with their daughters and facing foreclosure proceedings against their home.

She filed for Chapter 7 bankruptcy protection in July and for divorce in August. "It's very upsetting," she said in a brief interview, which she ended in tears.

For his part, Mr. Richard looks like a man who has long been resting. He has lost weight and added tattoos. He has a rich tan. Now and then he talks about book or movie deals. He takes precautions, but they are almost comically half-hearted, like his tendency when venturing away from the police to wear a hat and sunglasses, or his request to a reporter not to describe his tattoos.

The Luccheses' latest racketeering case, and his probation, will be over one day, he says. Then he will have a new life. "The last thing anybody wants to be is a rat," he said. "But you know something? I'll be a rat and I won't be in prison. I'll be a rat and I'll be alive."